[PowerPC] Work around -O miscompile on powerpc 32 bit.

Work around llvm 11 miscompile in 32 bit powerpc that appears to cause ifuncs
to branch to the wrong address by forcing -O2. This worked in previous
versions because -O was mapped to -O2 previously (but is now -O1.)

While here, remove the old temporary workaround from r224882 that does the
opposite thing for powerpc non-DEBUG kernels, bringing it in line with other
platforms that compile at -O2.

This fixes kernel boot on powerpc and powerpcspe after the llvm11 transition.

Sponsored by: Tag1 Consulting, Inc.